Add the ability to set the workqueue size

Part 1 of 2 to help address https://github.com/adhearsion/blather/issues/139

* Allow users to set the size of the GirlFriday::WorkQueue by setting
  Blather::Client.queue_size.
* The queue size is copied from the class at instance initialization time,
  so it is possible to make clients with different queue sizes if necessary.
* If the queue size is 0, then #receive_data will process the incoming stanza
  immediately.

Note: as the initializer has no arguments, we felt a class method was the best option here. I figure the most likely case is that a given application will want the same queue size for all of its clients, but if there's a better way to specify that, please let me know.
That really just moves the conditional from here to where the default is set (at least, so long as we avoid building a `HandlerQueueFactory`...), but if you like hiding that in `handler_queue`, I can see that looking less obtrusive, at least when you're following the flow of `receive_data`.